1. Field of the Invention
The present invention relates to a process for producing a stable acidic aqueous alumina sol containing 50 to 300 nm of elongate secondary particles which are elongated in only one plane and formed by edge-to-edge coagulation of rectangular plate-like primary particles having a length of one side of 10 to 30 nm when observed through an electron microscope.
The present invention also relates to a process for producing a stable acidic aqueous alumina sol containing colloidal particles of an alumina hydrate having a boehmite structure.
2. Description of the Related Art
Various colloidal particles of alumina hydrates have already been produced by various processes for production. Most of these produced colloidal particles are composed of alumina hydrates having a boehmite structure or pseudoboehmite structure, or amorphous alumina hydrates, and it is known that they are plate-like, ribbon-like, spindle-like, needle-like or fibrous in shape.
Japanese Patent Laid-open Nos. Sho 54-116398, Sho 55-23034 and Sho 55-27824 disclose a process for producing an aqueous alumina sol containing plate-like colloidal particles of an alumina hydrate having a boehmite structure, which comprises reacting a basic aluminum salt with an acid or alkali or reacting an acidic aluminum salt with an alkali to obtain an alumina gel and peptizing the alumina gel with an acid.
Japanese Patent Laid-open No. Sho 55-116622 discloses a process for producing a suspension containing alumina hydrate particles having a boehmite structure and various shapes such as plate-like, ribbon-like and fibrous shapes, which comprises treating poorly crystallized and/or amorphous activated alumina powder in an aqueous medium having a pH of 9 or less at a temperature of 120 to 225.degree. C.
Japanese Patent Laid-open No. Sho 58-176123 discloses a process for producing a suspension containing alumina hydrate particles having a pseudoboehmite structure or boehmite structure and various shapes such as plate-like, ribbon-like and fibrous shapes, which comprises adding an amorphous hydroxy aluminum carbonate gel obtained by reacting a carbon dioxide gas with alkali aluminate to a solution of an acid, base, salt or mixture thereof, adjusting pH of the medium of the resulting mixture to 11 or less, heating it at a temperature of not higher than 90.degree. C. for at least 5 minutes and further heating at a temperature of 90 to 250.degree. C.
A process for producing an alumina sol which comprises hydrolyzing aluminum alkoxide with water and peptizing the obtained slurry with an acid is described in American Ceramic Society Bulletin, Vol. 54, No. 3, pp. 289, 1975, athesis written by B. E. YOLDAS. This process is characterized by hydrolyzing conditions including a temperature of 75.degree. C. and use of water in a proportion of about 100 moles based on 1 mole of the aluminum alkoxide and peptizing conditions including a temperature of not lower than 80.degree. C. and use of hydrochloric acid or nitric acid as a peptizing agent. The obtained sol is a transparent aqueous alumina sol containing plate-like colloidal particles of an alumina hydrate having a boehmite structure.
Japanese Patent Publication No. Sho 45-3658 and Japanese Patent Laid-open No. Sho 60-166220 disclose a process for producing an aqueous alumina sol containing fibrous colloidal particles of an amorphous alumina hydrate, which comprises reacting an aqueous solution of an organic acid with metal aluminum powders under heating.
Japanese Patent Laid-open No. Hei 5-24824 teaches a process for producing an aqueous alumina sol composed of secondary particles having a length 5 to 10 times its thickness, which comprises reacting an aqueous solution of an acid such as a hydrochloric acid with metal aluminum in the presence of a trace amount of silicic acid ions and a trace amount of sulfuric acid ions to produce colloidal particles of an alumina hydrate. The secondary particle consists of an aggregate of fine fibrous primary particles of an amorphous alumina hydrate.
As described above, most of the colloidal particles of alumina hydrates obtained by various processes are composed of aluminate hydrates having a boehmite structure, pseudoboehmite structure or amorphous structure and it is known that they are plate-like, ribbon-like, spindle-like, needle-like or fibrous in shape.
An aqueous alumina sol in which colloidal particles of an alumina hydrate are stably dispersed is used in the surface treatment of fibers and paper, precision casting and the production of binders for refractories and catalysts.
The required physical properties such as viscosity and thixotropic property of the aqueous alumina sol are different according to each application purpose and it is desired to provide a further improved aqueous alumina sol.